There is a continuing desire in the microelectronics industry to increase the circuit density in multilevel integrated circuit devices such as memory and logic chips in order to improve the operating speed and reduce power consumption. In order to continue to reduce the size of devices on integrated circuits, it has become necessary to use insulators having a low dielectric constant (k) to reduce the resistance-capacitance (“RC”) time delay of the interconnect metallization and to prevent capacitive crosstalk between the different levels of metallization. Such low dielectric materials are desirable for premetal dielectric layers and interlevel dielectric layers.
Typical dielectric materials for devices with 180 nm line width are materials with a dielectric constant between about 3.8 and 4.2. As the line width decreases, the dielectric constant should also be decreased. For example, devices with 130 nm line width require materials with a dielectric constant between about 2.5 and 3.0. Extremely low dielectric constant (“ELK”) materials generally have a dielectric constant between about 2.0 and 2.5. Devices with 90 nm line width require materials with dielectric constants less than 2.4.
One approach to lowering the dielectric constant of interlevel dielectrics used in integrated circuit manufacture is to introduce porosity into the film. However, as the pore size approaches the dimensions of the features on the chip, the probability for failure increases. It is desirable to keep the pore dimensions smaller than the lateral dimensions or feature size in the interlayer dielectric films in order to minimize defects due to shorting via metal ion migration or dielectric breakdown. Typically, the metal migration is caused by defects in the barrier layers, which allow copper to diffuse into the dielectric layer. With more advanced barrier schemes, the IC manufacturers want to use CVD methodologies with smaller molecular precursors to deposit the barrier layer, therefore the pore size in the dielectric film should be small enough to insure that the barrier itself will not diffuse into the porous structure. The term “killer pores”, as used herein, describes pores that that can cause catastrophic failure. Because these defects may result in production loss, there is a need for measuring the pore size distribution for low κ films used for interlevel dielectrics.
Historically, volumetric and gravimetric adsorption techniques have been extremely valuable for characterizing the pore size and pore volume of mesoporous materials in the form of powders. Static volumetric adsorption techniques are those in which quantities of adsorbate gas are admitted from a manifold to an attached adsorption cell holding the sample at a controlled temperature, and the amounts of gas adsorbed at equilibrium are calculated using mass balance equations from the measured pressures before and after the dose is delivered, the free space values for the manifold and the sample holder, and appropriate gas equations of state. Gravimetric adsorption techniques are those in which the amounts of gas adsorbed at controlled pressures and temperatures are measured by the change in weight of the sample. The adsorption isotherm is converted to a pore size distribution using a model for capillary condensation that relates the pressure of adsorption to the size of the pores. However, these techniques may be difficult to use when the materials are in the form of thin films on substrates such as a silicon wafers. These thin films have too small a total pore volume and surface area to provide sufficient sorption for analysis by volumetric or gravimetric adsorption. Consequently, certain porosimetric techniques, such as N2 adsorption, mercury intrusion, or quartz crystal microbalances, may lack the sensitivity to characterize porosity in films that are less than 1 μm thick and attached to a 1-mm-thick silicon single-crystal wafer.
To remedy this, films are scraped off several substrates or wafers to provide a sufficient sample for analysis. Scraping the film off the substrate, however, could alter the pore size distribution. Further, when the pore size distribution is determined by adsorption techniques involving capillary condensation, the resulting powder can generate “phantom” porosity because of a sharp upturn in the isotherm at high relative pressure (P/P0) resulting from capillary condensation in the interstices between the powder particles. This phantom porosity cannot be distinguished from the presence of large pores in the pore size distribution calculations that would be attributed to killer pores. Consequently, there is a need for characterizing the film while still on the wafer in order to determine whether killer pores are present in the film.
Even when adsorption techniques are used to characterize the film on the wafer, the adsorption isotherm typically has a gradual positive slope at high relative pressures. It is not clear whether the gradual slope in the isotherm is caused by adsorption in pores having a large diameter. However, when classical models for capillary condensation are used to convert the adsorption isotherm to a pore size distribution, this gradual uptake at high P/P0 is ascribed to uptake by larger pores. The use of classical models may limit the sensitivity to the absence of killer pores. Thus, there is a need for higher sensitivity to the absence of killer pores, or “a lower killer pore volume absence detection limit” as used herein.
Several advanced non-destructive methods have been pursued by the semiconductor industry that measure the porosity of the film while still on the wafer as an alternative to the aforementioned analytical methods. Some of these alternative techniques use a beam of radiation rather than adsorption. Positronium annihilation lifetime spectroscopy (“PALS”) correlates the pore size with the lifetime of positronium (“Ps” which is the electron bound state with its antiparticle the positron), which is created when a beam of positrons is focused on the film, and which decays through collisions with bound electrons in the pore walls. PALS requires a positron beam source and gamma ray detector. In addition, films with open connected porosity may need to be capped with a non-porous layer prior to PALS analysis.
In other techniques such as small-angle neutron scattering (“SANS”) and small-angle X-ray scattering (“SAXS”), an average chord length or pore size is mathematically extracted from the plot of scattering intensity versus scattering vector using a model for the void and solid phases in the structure. More recently, radial diffuse X-ray reflectivity (“XRR”) has been used to measure pore size. In the radial diffuse XRR analytical technique, a radial diffuse XRR scan is obtained at grazing incidence angles by impinging the incident X-ray beam at an angle between the critical angle of the film and the critical angle of the substrate and scanning the detector over a large range to capture scatter as a function of angle. The resulting data can be modeled to determine the pore size by, for example, using a 1- or 2-parameter model of a standard distribution of pores around the average pore diameter because of the paucity of data. The above techniques—SANS, SAXS, and XRR—use relatively sophisticated equipment to induce scattering in order to measure the pore size of the substrate on one spot of the substrate at a time. These techniques also assume a shape for the distribution of pores around an average pore diameter.
Still other alternative analytical techniques may measure the porosity of the film while still on the wafer using adsorption techniques besides volumetric or gravimetric adsorption techniques to monitor the uptake of a reference vapor or gas onto an adsorbent. In this connection, the surface acoustic waves (“SAW”) technique has been used to determine the extent of adsorption of, for example, N2 at 77 degrees Kelvin (“K”), on mesoporous films deposited on a special piezoelectric substrate. The SAW technique may use the change in the oscillation frequency of surface acoustic waves as a function of change in mass density to measure the amount adsorbed. Yet another technique, the ellipsometric porosimetry (“EP”) technique, measures the extent of adsorption of an organic vapor, such as toluene at room temperature, by the change in the refractive index. The EP analytical technique assumes the index of refraction of a condensed liquid (e.g., toluene) in a nanoscale pore is identical to that of the bulk liquid. Although the small analytical area of less than 1 mm3 may be appropriate for the microelectronic industry, particularly if it is desirable to “isolate” the presence of defects on a specific location on the wafer, it may be impractical as a quality control tool in identifying whether there are any defects present on an entire wafer. The X-ray porosimetry (“XRP”) analytical technique monitors the extent of uptake of a reference fluid, such the organic vapor toluene, on exposure to the adsorbate by change in the critical angle of reflection measured by X-ray reflectivity, which depends on the change in density of the film. The XRP method requires the atomic composition of the film to convert electron density to mass density, which usually requires the use of complex techniques such as forward recoil elastic scattering and Rutherford backscattering. Calculations of pore size distribution from adsorption isotherms measured with the aforementioned analytical techniques—SAW, EP, and XRP—have traditionally used historical procedures, such as those procedures advanced by Barrett, Joyner, and Halenda, and/or classical theoretical models of capillary condensation such as the Kelvin equation.
In processes for producing porous films substrates, there may be a need for quality control testing in order to accept or reject films based on the presence of killer pores. Further, there may also a need for process control in the film production process in order to change the parameters of the film production process based on the presence of killer pores.
All references cited herein are incorporated herein by reference in their entireties.